improve iterDeps in some scenarios by 20-30% in v8, no noticeable dow…

…nsides in other evergreen browsers

1. don’t enumerate keys, if no deps exist to produce keys
2. in v8 for (x in y) where y is non enumerable deopts entire function [readmore](https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#5-for-in)
3. minor: avoid deep property lookups like x[y][z] if we already have a reference to x[y]

packages/ember-metal/lib/property_events.js

@@ -100,28 +100,36 @@ function dependentKeysDidChange(obj, depKey, meta) {
   if (top) { DID_SEEN = null; }
 }
 
+function keysOf(obj) {
+  var keys = [];
+  for (var key in obj) {
+    keys.push(key);
+  }
+  return keys;
+}
+
 function iterDeps(method, obj, depKey, seen, meta) {
-  var guid, deps, keys, key, i, desc;
-  guid = guidFor(obj);
-  if (!seen[guid]) seen[guid] = {};
-  if (seen[guid][depKey]) return;
-  seen[guid][depKey] = true;
+  var deps, keys, key, i, desc, descs;
+  var guid = guidFor(obj);
+
+  var current = seen[guid];
+  if (!current) current = seen[guid] = {};
+  if (current[depKey]) return;
+  current[depKey] = true;
 
   deps = meta.deps;
   deps = deps && deps[depKey];
-  keys = [];
   if (deps) {
-    for(key in deps) {
-      keys.push(key);
+    keys = keysOf(deps);
+    descs = meta.descs;
+
+    for (i=0; i<keys.length; i++) {
+      key = keys[i];
+      desc = descs[key];
+      if (desc && desc._suspended === obj) continue;
+      method(obj, key);
     }
   }
-
-  for (i=0; i<keys.length; i++) {
-    key = keys[i];
-    desc = meta.descs[key];
-    if (desc && desc._suspended === obj) continue;
-    method(obj, key);
-  }
 }
 
 function chainsWillChange(obj, keyName, m) {